The structure of a conventional aluminum-gallium-indium-phosphide (AlGamnP) light-emitting diode is shown in FIG. 1. An n-type gallium-arsenide (GaAs) substrate 11 has a bottom electrode 10. An n-type AlGaInP cladding layer 12 is grown over the substrate and is grown with an undoped AlGaInP active layer 13. A p-type AlGaInP cladding layer 14 is grown over layer 13. Then a p-type gallium-phosphide (Ga-P) layer 15 is grown over layer 14. A p-type electrode 16 is deposited over layer 15.
The energy bandgap of the AlGaInP is proportional to the aluminum mole fraction. When the aluminum mole fraction in the cladding layer is larger than the mole fracion in the active layer 13, the confinement effect of the electrons is better. In the past study of the AMGaInP material, it was assumed that when (AlxGa1-x)0.5In0.5P is lattice-matched to GaAs substrate, the material can be changed from direct bandgap into indirect bandgap with a cross-over point of x=0.7. The relationship between the direct bandgap and the value of x is given by the following formula: EQU Eg(x)=1.91+0.61x(eV) (1)
Researchers found that the degree of electron confinement is related to the energy and gap. In general, the conduction band offset (.DELTA.Ec) is proportional to energy band. offset (.DELTA.Eg). in other words, researchers considered that the maximum .DELTA.Ec occurs when x=0.7, i.e. the confinement effect in the double heterojunction AlGaInP is at its maximum when x=0.7. However, recent researchers found that the cross-over point when the material changes from direct band gap to indirect band gap occurs at a lower value of x=0.7. When this double-beterojunction AlGaInP material is used to obtain short wavelength yellow-green or pure green light emitting diodes, and the light emitting efficiency is low. This observation further made researchers to doubt whether the transition point from direct band to indirect band gap occurs at of value of x lower than x=0.7. Recently, D. I. Mowbvey and A. D. Print found that the transition point from direct bandgap to indirect bandgap of AlGaInP occurs when the aluminum fraction (xAl) lies in the range 0.5.about.0.53. Therefore, the maximum direct band gap from equation (1) is 2.23 eV instead of 2.34 eV. If .DELTA.Ec/.DELTA.Eg=0.6, the following table shows the difference in electron confinement effect for different values of x.
______________________________________ Electron confinement Active layer AL fraction Cladding layer Al fraction (meV) ______________________________________ x = 0.3 (.lambda. .about. 592 nm) x = 0.7 64 x = 0.53 85 x = 0.45 (.lambda. .about. 568 nm) x = 0.7 9 x = 0.53 30 ______________________________________
From the above table, we know that when AlGaInP is used as a cladding layer, the confinement effect is not good, especially for the top p-type confinement layer.